This invention relates to an improved scanning electron microscope (hereinafter called SEM) having a field emission gun (hereinafter called FEG) and more particularly relates to a device for correcting the influence of the current fluctuation of an electron beam in a SEM having FEG (hereinafter called FE-SEM).
As is well know, the electron beam brightness of the virtual source formed by a FEG is considerably higher than that of a thermal emission electron gun. In FEG, an emitter tip is maintained at a voltage V0 for accelerating or decelerating; the electron beam emitted from between the emitter tip and an anode maintained at ground potential. An extraction electrode facing the emitter tip is maintained at extraction voltage V1 for forming the strong electric field near the emitter tip. The extraction voltage V1 is so adjusted that optimum field emission at the emitter tip is carried out. The position of the virtual source of FEG is varied according to the electrostatic lens field formed between the anode and the extraction electrode. For example, under the condition of V0&lt;V1, the position of the virtual source is shifted along the optical axis far away from the anode by increasing the acceleration voltage V0 or voltage ratio r (=V0/V1). Thereby, the current intensity of the electron beam passing through the anode increases as the distance between the virtual source and the anode increases.
On the other hand, in a FEG, the current intensity of the electron beam is apt to fluctuate under the influence of the frequent absorption and desorption of residual gas molecules or ion bombardment of the surface of the emitter tip. Consequently, the current intensity of the electron beam passing through the anode consists of a constant component and a fluctuation (or noise) component. The current fluctuation of the electron beam irradiating a specimen causes noise to form in the output signal of the detector which detects the secondary electrons yielded from the specimen. The noise in the output signal of the detector causes a brightness fluctuation in a normal microscope image displayed in a SEM, and causes to flicker noise in the scanning image. Because of this, an ordinary FE-SEM incorporates a compensation device for detecting the fluctuation of the electron beam and compensating the influence of the fluctuation component of the electron beam included in the secondary electron signal from a specimen. This type of compensation device in the FE-SEM is disclosed in U.S. Pat. No. 3,783,281. This device provides sufficient compensating effect in the cases where an acceleration voltage for the electron beam is in a narrow operating range, e.g., 5 kV -30 kV, and where an electron beam current detected as a monitor signal is as large as, for example 1.times.10.sup.-9 Amperes or more.
Recent cases of observing and analyzing various materials require the acceleration voltage V0 to be in a wide operating range of 0.5 kV-50 kV. In the cases, the current intensity of the electron beam is also varied within a very wide range, e.g., 1.times.10.sup.-8 Amperes -.times.10.sup.-10 Amperes. However, when the input signal indicating the fluctuation of the electron beam is over or under the critical value within said wide range, the frequency characteristic of the conventional compensation device deteriorates or the output of the conventional compensation device saturates. For this reason, the conventional compensation device cannot provide sufficient compensating effect when accelerating voltage is changed in said wide range. The FE-SEM for resolving this problem is described in U.S. Pat. No. 4,588,891. In this type of device, two detecting means for detecting current of the electron beam are disposed at different positions in the optical system in SEM and either one of the two means is selectively used according to the accelerating voltage. However, this type of device increases cost of manufacture and intricacy of maintenance and adjustment.